Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc.
In addition to basic chemical reactions promoted by medium-pore zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain olefins. Conversion of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 4,760,024) and Yan et al. (U.S. Pat. No. 3,845,150) to be effective processes using zeolite catalyst having the structure of ZSM-5. The '150 patent to Yan et al. teaches a heat-balanced process for producing aromatic gasoline. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
U.S. Pat. No. 3,759,821 to Brennan et al. teaches a process for the catalytic upgrading of a cracked gasoline which involves fractionating a catalytically cracked gasoline into a C.sub.6 -overhead and a C.sub.7 + bottom fraction and contacting the C.sub.7 + bottom fraction with a catalyst having the structure of ZSM-5.
Conversion of olefins, especially alpha-monoalkenes such as propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5 + aliphatic and aromatic hydrocarbons. Product distribution for liqud hydrocarbons can be varied by controlling process conditions, such as temperature, pressure and space velocity. Aromatic gasoline (C.sub.5 -C.sub.10) is readily formed at elevated temperature (e.g. about 425.degree. to 650.degree. C.) and moderate pressure from ambient to about 5500 kPa, preferably about 200 to 2900 kPa. Olefinic gasoline can also be produced and may be recovered as a product or fed to a low severity, high pressure reactor system for further conversion to heavier distillate range products or otherwise utilized. Operating details for typical "MOGD" oligomerization units are disclosed in U.S. Pat. Nos. 4,456,779; 4,497,968 (Owen et al.) and 4,433,185 (Tabak), incorporated herein by reference.
In MOGD and MOGDL, olefins are catalytically converted to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as a zeolite catalyst having the structure of ZSM-5. Process conditions can be varied to favor the formation of either gasoline, distillate or lube range products. U.S. Pat. Nos. 3,960,978 and 4,021,502 to Plank et al. disclose the conversion of C.sub.2 -C.sub.5 olefins alone or in combination with paraffinic components, into higher hydrocarbons over a crystalline zeolite catalyst. U.S. Pat. Nos. 4,150,062; 4,211,640 and 4,227,992 to Garwood et al. have contributed improved processing techniques to the MOGD system. U.S. Pat. No. 4,456,781 to Marsh et al. has also disclosed improved processing techniques for the MOGD system. The conversion of olefins in an MOGDL system may occur in a gasoline mode and/or a distillate/lube mode. In the gasoline mode, the olefins are typically oligomerized at temperatures ranging from 200.degree. C. to 430.degree. C. (400.degree. F. to 800.degree. F.) and pressures ranging from 70 kPa to 6900 kPa (10 to 1000 psia).
U.S. Pat. No. 4,090,949 to Owen and Venuto teaches a process for upgrading olefinic gasoline by recycling FCC gasoline to a second FCC riser together with a stream of light C.sub.2 -C.sub.5 olefins which serve as hydrogen contributors. The processing scheme disclosed in the '949 patent recycles gasoline through the FCC gas plant thereby increasing both capital and operating costs associated with the gas plant. Further, recycling gasoline to the riser of a catalytic cracking unit exposes the gasoline to severe temperature conditions which promote cracking and tend to decrease gasoline yield. Thus it can be seen that it would be highly desirable to provide a process for upgrading highly olefinic gasoline produced in a catalytics cracking process while at the same time utilizing the existing catalytic cracking unit gas plant to separate the upgraded gasoline product.